S4 not running well no more...

[Bambi]

My UR S4 used to make 165kW on the wheels with a chip and exhaust mod. Then one day the car began running inconsistent. Losing about 30kW on some runs. If I shift the power is less, then I quickly lift and plant my foot again and the power is back.

The other day I suddenly had a misfire. Nice and constant but sounding like 2 cylinders missing. I went to an Audi dealer who "checked" the TCi and coils by unplugging the injectors one by one to hear if there was a diffenrence - the car was missing at idle as well. Eventually they said coils. I swithed the car off for about and half hour and then started to go home. Suddenly the miss was way less. On the way home the miss almost dissappeared except for missing under boost. I changed the spark plugs (to A3 1.8T plugs - I had them lying around) to see if that solved the problem- No. I then went to a friend who also has S4 and borrowed his coils. The car ran beautifully. I stopped, put my coils back and suddenly my miss was completely gone, except that now I'm down to 135kW on the wheels (according to Gtech) I borrowed the other coils again to measure and : down to 125kW. Now I'm thinking maybe TCi unit giving a problem.

How do I test the coils and how do I test the TCi units? Any help will be greatly appreciated.

[elijahallens2]

Sounds to me like a bad POS. They are usually intermittent when they fail. See if you can borrow one from your friend cause they are not cheap.

Elijah

[Bambi]

POS - is that the throttle position sensor?

[elijahallens2]

Position output stage. There are two of them on the firewall. They are what control the firing order of the coils.

ELijah

[S2central.net]

POS = Primary Output Stage. The little ignition amplifiers for the coil pack. Like the ABY, the AAN runs a pair of these. Three channels in each unit. It looks like a TCi unit of old so I guess you are talking about the same thing.

Coilpacks are hard to test unless you have the right equipment - a highly sensitive conductance measurement device.

POS units are easy to test by a replacement method. Always remember you are carrying a spare channel so by rewiring the POS connectors you can bypass a bad channel. You need to be absolutely certain which is dead of course. Doubtless there are some electrical tests you could make to compare all six available channels and you might detect a duff one. Haven't researched this yet though.

Coilpack failures certainly arent rare - seems to be more of them recently with age creeping in on them and doubtless some high mileage takes its toll


Paul

[Nuvo]

Okay we've had the POS defined as the Position Output Stage and the Primary Output stage, to further confuse the issue I believe it is actually called Power Output Stage. Otherwise good info.

See these links for some tasty troubleshooting info:

http://www.sjmautotechnik.com//trouble_shooting/S4_Ignition.htm

and

http://www.audifans.com/twiki/bin/view/Audi/PowerStageOutputRepair

That should help you. Oh and if you find one to be faulty, then they are more commonly referred to as a Piece Of Shi..
Nuvo.

[elijahallens2]

Okay we've had the POS defined as the Position Output Stage and the Primary Output stage, to further confuse the issue I believe it is actually called Power Output Stage. Otherwise good info.

See these links for some tasty troubleshooting info:

http://www.sjmautotechnik.com//trouble_shooting/S4_Ignition.htm

and

http://www.audifans.com/twiki/bin/view/Audi/PowerStageOutputRepair

That should help you. Oh and if you find one to be faulty, then they are more commonly referred to as a Piece Of Shi..
Nuvo.

LOL, yea, that sounds right, or what we call them here in the states, POS= piece of ****

Elijah

[Bambi]

Thanx for the info. i checked them by swapping them out : no change. I did however put new genuine plugs in and the slight mis at idle dissappeared. Under load it is still another story. It feels like the car is either dropping one cylinder on and off between 4000 and 6000 rpm or like the power is opened up and closed all the time, much like a stepped boost curve on some cars I have driven like the volvo s40 T4. It is quite hot here these days about 33deg C, but even so, the car actually feels stronger during the day than at night. It also has a tendency to over boost. I'm supposed to run 1.4Bar but if I nail it at 3000rpm, it now most of the time shoots up to 1.6- 1.7 bar in which case the management pulls back the boost for a couple of seconds. My wastegate setting is fine (i.e. 0.4bar with the N75 disconnected). I stripped the wastegate to check for stickyness but all is fine. I'm not sure whether a faulty N75 couls cause that. Maybe I should move the wastegate exhaust to it's own silencer?

Could the TPS (throttle position sensor) be at fault? How do you clean them? I notice that during tapping off at most speeds in most gears if I hold the throttle for constant speed or just trailing throttle the car has an uneven rumble and sometimes even jerks. I do have a funny jerk between shifts sometimes but it is hard to trace cause it is not always present. It drives me nuts though.....

[S2central.net]

Air leak then perhaps - you are drawing excess air in... and if gear changes are funny I'd check the bypass valve and associated control tubing.

Paul

[elijahallens2]

Also, If the A3 plugs are not the same as the urs4 plugs, put the right plugs in, they are the only ones that work!

Elijah

[mastermind]

I intend to agree with Paul, next step is to check for boost leaks